The present invention relates to an improved method and apparatus for echo cancellation in communication systems utilizing bidirectional transmission media such as copper wire twisted pairs, air, water, and outer space. In particular, the invention relates to a method and apparatus for echo cancellation in a communication system utilizing different signaling or baud rates or different spectral shapes in the transmit and receive directions, whereby the computational overhead of the echo cancellation is significantly reduced as compared to conventional methods. In preferred embodiments described herein, the present invention obtains benefits not obtainable in the prior art, e.g., the present invention reduces by one-half the computational overhead associated with echo-cancellation in a data communications system utilizing symmetrical information rates at asymmetrical signal rates.
Signals, i.e., the intelligence, message or effect conveyed over a communication system, are susceptible to the unwanted effects of echoes. Echoes are those signal components that represent a delayed and distorted version of the original transmitted signal reflected back into the source or a receiver near the source. Consequently, echoes are superimposed upon useful signals, thus distorting the useful signal and impairing communication quality.
For example, in an acoustic system comprised of a microphone and a loudspeaker, echoes cause the received signal at the microphone to be distorted. This distortion occurs as a result of acoustic coupling between the microphone and the loudspeaker, and detection by the microphone of reflections of the sound waves generated at the loudspeaker.
Likewise, signals transmitted over telephone lines in a full-duplex mode, i.e., signals transmitted in two directions simultaneously, are susceptible to the effects of echo. In typical telephone networks comprised of two-wire loops (the "subscriber loops") and four-wire loops (the "long-haul" loops) echo distortion is caused by the mismatch between the characteristic impedance of directional couplers, or hybrids, and the input impedance of the line. In such a system, the two-wire loop is used for full-duplex transmission, whereas the four-wire loop (comprised of two separate two-wire loops) is used for half-duplex transmission. These loops are coupled together by directional couplers such as two-wire to four-wire ("2W/4W") hybrid coupling circuits located at the near and far ends of the two-wire loops.
However, because of inherent impedance mismatches between the couplers and the telephone lines, directional couplers contribute two forms of echoes, i.e., near-end echoes and far-end echoes. Near-end echoes are generated by energy leaking through the first hybrid encountered by the transmitted signal. For example, in communication systems comprised of remote site ("upstream") and central office ("downstream") site transceivers, near-end echoes are caused by transmit energy leaking across the remote site hybrid into the remote site receiver, thereby corrupting the information received at the remote site. Near-end echo is further characterized by high amplitude and short time delay.
Far-end echoes on the other hand are generated by energy leaking through the hybrid at the far end of the circuit. In the example discussed above, a mismatch between line and hybrid impedances at the central office site causes remote site transmitted energy to leak across the central office hybrid. This energy is reflected back into the remote site receiver along with the incoming signal transmitted from the central office. In contrast to near-end echo, far-end echo is characterized by weaker amplitude and longer time delay.
Consequently, echo cancelers have been used to minimize the effects of echo distortion in communication systems susceptible to echo. Examples of such systems include full-duplex, two-wire telecommunication systems. See Jean-Jacques Werner, "An Echo Cancellation-Based 4800 Bit/s Full-Duplex DDD Modem," IEEE Journal on Selected Areas in Communications, vol. SAC-2, no. 5, p. 722 (September 1984); Loic Guidoux, "Echo Canceling on Full-Duplex Dial-Up Modems," Data Communications, p. 227 (September 1983).
Echo cancelers in these and other systems operate by subtracting a replica of the echo of the original signal from the received signal. In the above-mentioned remote site-central office system example, the echo component of the incoming signal at the remote site is replicated from the transmitted signal at the remote site. The echo can be replicated by first storing in the echo canceler each of a predetermined number of previous consecutive symbols in the transmitted signal. Each symbol is then multiplied by a respective tap coefficient set during training of the echo canceler. The resulting products are summed to produce the replicated echo, which in turn is subtracted from the incoming signal at the remote site receiver input. The resulting echo compensated signal is then processed to retrieve the desired information from the received signal.
For the replicated echo to be properly subtracted from the received signal, the canceler must be operated such that replication, sampling of the incoming signal, and echo cancellation (subtraction of the replication from the incoming signal) occur at the same signal rate. Consequently, for fully symmetrical information services, echo cancelers have been operated at the baud rate, i.e., the symbol or signal rate, and at the Nyquist rate, i.e., a rate equal to at least twice the highest possible frequency present in the incoming signal. See U.S. Pat. No. 4,087,654, "Echo Canceler for Two-Wire Full-Duplex Data Transmission," issued to K. H. Mueller; U.S. Pat. No. 4,535,206, "Echo Cancellation in Two-Wire Full-Duplex Data Transmission With Estimation of Far-End Data Components," issued to D. D. Falconer.
For fully asymmetrical services such as Asymmetric Digital Subscriber Line ("ADSL" service) replication, sampling, and cancellation have been based on the transmit sample clock rate, i.e., the transmit rate of the central office. See "Echo Cancellation for Asymmetrical Digital Subscriber Lines," IEEE Int'l Conference on Communications, p. 301 (May 1994). In such as system, the echo compensated signal is subsequently interpolated to generate the appropriate receiver samples. However, this method is computationally inefficient at the remote site since echo cancellation is performed over the wider bandwidth of the central office transmit signal.
Consequently, the ADSL solution emphasizes the major shortcoming of traditional echo cancellation methods embodied in the prior art. These methods can be unnecessarily computationally intensive and costly.
Prior art echo cancelers are also inefficient in communication systems utilizing, for example, symmetric information rates at asymmetric signaling or baud rates or with asymmetrical spectral shapes. Such communication systems have been recently proposed for the single-pair High-Bit-Rate Digital Subscriber Loop ("HDSL2"). See Kevin Schneider and Richard Goodson, "A Modulation Technique for CSA Range HDSL2," T1E1.4 Technical Subcommittee Working Group, T1E1.4/97-xxx(February 1997) and M. Rude, H. Taktori, and G. Zimmerman,A proposal for HDSL2 Transmission: OPTIS, T1E1.4/97-238 (Jun. 30-Jul. 2, 1997).
It is, therefore, a principal objective of the present invention to provide a new method and apparatus for echo cancellation in a communication system utilizing a bidirectional transmission medium.
It is another object of the present invention to provide a new method and apparatus for echo cancellation in a communication system utilizing different signaling or baud rates in the transmit and receive directions.
It is another object of the present invention to provide a new method and apparatus for echo cancellation in a communication system utilizing symmetrical information rates at asymmetrical signal rates.
It is yet another object of the present invention to provide a new method and apparatus for echo cancellation that are characterized by significantly reduced computational overhead as compared with conventional methods and conventional apparatus.
It is a further object of the present invention to provide a significantly more efficient and economical method and apparatus for echo cancellation in an HDSL2 system.